System and method for electrochemical processing of non-flat samples

ABSTRACT

A system and method for electrochemically processing non-flat samples and/or samples that can change shape during the electrochemical processing is disclosed. In one aspect, a system includes a sample holder for providing an electrical contact to the sample during electrochemical processing. The sample holder has a carrying element and a fixing element for clamping of the sample in between the fixing element and the carrying element, thus providing electrical contact to the sample while allowing the sample to change shape without interrupting the electrical contact.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT Application No.PCT/EP2010/060965, filed Jul. 28, 2010, which claims priority under 35U.S.C. §119(e) to U.S. provisional patent application 61/238,993 filedon Sep. 1, 2009. Each of the above applications is incorporated hereinby reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosed technology relates to methods and systems forelectrochemical processing of non-flat samples and/or samples that maychange shape during electrochemical processing, and more in particular,to methods and systems for holding such samples and for maintaining agood electrical contact to such samples during electrochemicalprocessing, e.g. electrochemical etching, plating or cleaning.

2. Description of the Related Technology

Nowadays' production of photovoltaic cells mainly relies on silicon. Theprice of silicon photovoltaic modules is to a large extent driven by thecost of the silicon material. Furthermore, with currently industriallyachievable surface recombination velocities, the optimum thickness forachieving the highest efficiency of a crystalline silicon photovoltaiccell is in the range of 50 micrometer. For a thickness below a few tensof micrometer, silicon becomes flexible. Encapsulated in a suitable andflexible material, it can provide a building block for applicationsrequiring lightweight, flexible, high efficiency photovoltaic modules.

When forming thin silicon wafers by wire sawing from an ingot, kerflosses consume half of the ingot and the achievable wafer thickness islimited to 100 micrometer or more. Therefore, other methods for formingthin silicon wafers for the photovoltaic industry are needed.

In US 2007/0249140 a wafering technique is described that enables theproduction of very thin (e.g. thinner than 100 micrometer) crystallinesilicon wafers. This process is also known as the SLIM-Cut(stress-induced lift-off method) process. In an embodiment described inUS 2007/0249140, on a thick mother substrate (e.g. silicon substrate) alayer (e.g. a metallic layer) with mismatched thermal expansioncoefficient with respect to the mother substrate is provided, e.g.screen printed. The thermal coefficient of expansion of the metalliclayer is much higher, e.g. more than a factor of 10 higher, than thethermal coefficient of expansion of silicon. Good bonding between themetallic layer and the silicon mother substrate can for example beachieved by a treatment at high temperature in a belt furnace. Uponcooling to room temperature, the metallic layer and the siliconsubstrate undergo a thermal contraction, and the mismatch in coefficientof thermal expansion between the metallic layer and the silicon inducesa large stress field in the mother substrate. Since the bonding betweenthe metallic layer and the silicon is sufficiently strong to withstandthis stress load, the stress builds up inside the silicon materialduring cooling down. When the stress reaches a threshold value, thesystem tends to relax by the initiation and the propagation of a crackparallel to the main surface of the mother substrate. After the crackhas completely propagated through the mother substrate, a film isdetached and the mother substrate can be reused. The film detached iscomposed of the metallic layer and a thin silicon substrate lifted-offfrom the mother substrate. The film is still under stress and exhibitstherefore some bending. In a next step the metal can (at leastpartially) be removed, resulting in a (flat) thin silicon film on whiche.g. a photovoltaic cell process can be applied.

The film detached from the mother substrate may be a rolled-up stackcomprising the thin silicon substrate and at least one layer of stressinducing material, e.g. metallic layer. As described in US 2007/0249140,the at least one layer of stress inducing material may be removed e.g.by chemical etching. By removing the at least one layer of stressinducing material the stress is relieved from the thin substrate and thethin substrate can then become flat and stress-free. For example, incase a stress inducing stack comprising a layer of Ag paste and a layerof Al paste is used, removing the Ag/Al layers after release of a filmfrom the mother substrate may comprise dipping the detached film in awarm HCl solution and in a mixture of HCl and HNO₃. However, when usingsuch a chemical etching process, Ag cannot be recovered easily.

SUMMARY OF CERTAIN INVENTIVE ASPECTS

Certain inventive aspects relate to a method and a system forelectrochemical processing of non-flat samples and/or samples that canchange shape during processing. The system comprises a sample holderthat provides an electrical contact to the sample during electrochemicalprocessing, wherein the holder allows the sample to change shape duringelectrochemical processing without interrupting the electrical contact.The system can be used for processing fragile samples with a low risk ofbreakage.

It is an advantage of electrochemical etching that it allows a bettercontrol of the etch rate and has the potential of higher etch rates ascompared to chemical etching, which may lead to faster processing. Inaddition, etching solutions may be used that are less aggressive than incase of chemical etching. In case of electrochemical etching, metalssuch as Ag can be recovered more easily than in case of chemicaletching.

One inventive aspect relates to a system comprising a sample holder, thesample holder comprising a carrying element and a fixing element adaptedfor being pressed against each other, thereby allowing clamping of asample, e.g., a substantially non-flat sample, in between the fixingelement and the carrying element. At least one of the carrying elementand the fixing element is electrically conductive and maintains anelectrical contact to the sample, enabling electrochemical processing.

The carrying element may be a grid-shaped element. It is an advantage ofa grid-shaped carrying element that during electrochemical processing itpermits a good flow of electrolyte towards the sample and it allows therelease of bubbles that may be formed during electrochemical processing.

In case the sample is bent in one direction, the fixing element may be alinear element, such that the contact area between the fixing elementand the sample is a linear area. It is an advantage of using a linearfixing element that it allows fixing a sample bent in one direction onthe carrying element and that it allows the sample to change shape whilebeing fixed on the carrying element. In case of a sample that is bent inmore than one direction, the fixing element may be such that the contactarea between the fixing element and the sample is a point or a verysmall area, e.g., a substantially circular area, wherein the dimensionsin one direction of the contact area are less than about 10% of thedimensions in that direction of the sample.

The carrying element (e.g. grid) may be such that it can be bent in afirst direction and in a second direction different from the firstdirection, wherein bending of the carrying element towards the seconddirection causes the carrying element to be pushed towards the fixingelement. The fixing element can be attached to the carrying element orit can be a separate element.

A system according to one inventive aspect can advantageously be usedfor holding a sample, e.g. a substantially non-flat sample or a samplethat can change shape during electrochemical processing, e.g.electrochemical etching, cleaning or plating.

Certain inventive aspects are set out in the accompanying independentand dependent claims. Features from the dependent claims may be combinedwith features of the independent claims and with features of otherdependent claims as appropriate and not merely as explicitly set out inthe claims.

Certain objects and advantages of certain inventive aspects have beendescribed herein above. Of course, it is to be understood that notnecessarily all such objects or advantages may be achieved in accordancewith any particular embodiment of the invention. Thus, for example,those skilled in the art will recognize that the invention may beembodied or carried out in a manner that achieves or optimizes oneadvantage or group of advantages as taught herein without necessarilyachieving other objects or advantages as may be taught or suggestedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a system according to an embodiment ofthe present invention showing a holder and a sample during introductioninto an electrochemical bath. FIG. 1( a) shows a cross-section along they-direction; FIG. 1( b) shows a cross-section along the x-direction.

FIG. 2 is a schematic drawing of a system according to an embodiment ofthe present invention showing a holder and a sample duringelectrochemical processing. FIG. 2( a) shows a cross-section along they-direction; FIG. 2( b) shows a cross-section along the x-direction.

FIG. 3 is a schematic drawing of a system according to an embodiment ofthe present invention showing a holder and a sample afterelectrochemical etching. A cross section along the x-direction is shown.

FIG. 4 is a schematic drawing of a ‘vertical’ system according to analternative embodiment of the present invention showing a holder and asample during electrochemical processing.

FIG. 5 shows a cross section of a grid with two stable positions (FIG.5( a): first position; FIG. 5( b): second position) that can be used forcarrying a sample in embodiments of the present invention.

FIG. 6 shows a flowchart of one embodiment of a method ofelectrochemically processing non-flat samples and/or samples that canchange shape during processing.

DETAILED DESCRIPTION OF CERTAIN ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto. The drawings described are only schematic and arenon-limiting. In the drawings, the size of some of the elements may beexaggerated and not drawn on scale for illustrative purposes. Thedimensions and the relative dimensions do not correspond to actualreductions to practice of the invention. In the different drawings, thesame reference signs refer to the same or analogous elements. Anyreference signs in the claims shall not be construed as limiting thescope.

Furthermore, the terms first, second, third and the like in thedescription, are used for distinguishing between similar elements andnot necessarily for describing a sequence, either temporally, spatially,in ranking or in any other manner. It is to be understood that the termsso used are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in thedescription are used for descriptive purposes and not necessarily fordescribing relative positions. It is to be understood that the terms soused are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other orientations than described or illustrated herein.

It is to be noticed that the term “comprising” should not be interpretedas being restricted to the means listed thereafter; it does not excludeother elements or steps. It is thus to be interpreted as specifying thepresence of the stated features, integers, steps or components asreferred to, but does not preclude the presence or addition of one ormore other features, integers, steps or components, or groups thereof.Thus, the scope of the expression “a device comprising means A and B”should not be limited to devices consisting only of components A and B.It means that with respect to the present invention, the only relevantcomponents of the device are A and B.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood bythose in the art.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

The invention will now be described by a detailed description of severalembodiments of the invention. It is clear that other embodiments of theinvention can be configured according to the knowledge of personsskilled in the art without departing from the true spirit or technicalteaching of the invention, the invention only being limited by theappended claims.

Certain embodiments of the present invention relate to a method and asystem for holding non-flat samples during electrochemical processing(e.g. electrochemical etching, electrochemical plating orelectrochemical cleaning), wherein the samples can be substantiallynon-flat and/or wherein the samples can change shape (e.g. become moreflat) during the electrochemical process. In one embodiment, the systemallows holding the non-flat samples with a low risk of breakage and itallows providing and maintaining an electrical contact to the samplesduring the whole electrochemical process. For example, if the sample isinitially rolled up, it can unroll during the electrochemical process,e.g. etching process, without losing the electrical contact. In oneembodiment, the system allows electrochemical etching or cleaning offragile samples with a low risk of breakage, for example of samples, forexample semiconductor material samples such as silicon samples, with athickness below 100 μm.

A system according to one embodiment of the present invention comprisesa sample holder comprising a carrying element such as e.g. a grid forcarrying and mechanically supporting the (substantially) non-flatsample, and a fixing element such as e.g. a wire or a rod to fix thesample on the carrying element e.g. during an electrochemical process,thereby allowing clamping the sample in between the fixing element andthe carrying element. At least one of the carrying element and thefixing element is electrically conductive and provides an electricalcontact to the sample during electrochemical processing. Duringelectrochemical processing the sample, e.g. non-flat sample, is locatedand fixed in between the carrying element and the fixing element.

The system is further described for a particular embodiment wherein thecarrying element is an electrically conductive element, e.g. a metallicgrid, carrying the (substantially) non-flat sample and providing anelectrical contact to the sample during electrochemical processing, andwherein the fixing element is electrically non-conductive. However, theinvention is not limited thereto.

The invention is further described for the exemplary embodiment whereinthe system is used for holding a sample comprising a thin silicon layer(e.g. a silicon layer with a thickness lower than 100 micrometer, e.g.lower than 80 micrometer, e.g. lower than 60 micrometer) and at leastone metallic layer, the sample being bent in one direction, and forelectrochemically etching the at least one metallic layer from thesilicon layer. Such a sample comprising a thin silicon layer and atleast one metallic layer may for example result from a SLIM-cut process(as described in US 2007/0249140, which is incorporated herein byreference in its entirety). Before further processing of such a sampleafter release from a mother substrate, e.g. for making a photovoltaiccell in the thin silicon layer, the at least one metallic layer needs tobe removed at least partially. Certain embodiments of the presentinvention relate to methods and systems for removing the at least onemetallic layer by electrochemical etching. Due to stress in thesilicon/metal sample, the sample is initially curled or rolled up, e.g.in a cylindrical shape. During the etching process the stress isreleased and the sample may change its shape, e.g. from a cylindricalshape to a substantially flat or flat shape or to a cylindrical shapewith a larger radius. In one embodiment, a sample holder allowsuncurling of the sample during the electrochemical process, e.g.electrochemical etching process. During the electrochemical process anelectrical contact to the sample is provided, and it is maintained evenwhen the sample is changing shape, e.g. when it is uncurling.

US Patent Application 2007/0249140 is incorporated herein by referencein its entirety. To the extent this patent application contradicts thedisclosure contained in this specification, this specification isintended to supersede and/or take precedence over any such contradictorymaterial.

FIG. 1 and FIG. 2 show schematic drawings illustrating a systemaccording to an embodiment of the present invention, wherein a curledsample 1 comprising a thin silicon layer and at least one metal layercan be etched, i.e. wherein the at least one metal layer can be removedfrom the sample. FIGS. 1( a) and 2(a) show a cross section of the systemalong the y-direction, the y-direction being a direction in which thesample is not bent or curled. FIGS. 1( b) and 2(b) show a cross sectionof the system along the x-direction, the x-direction being a directionin which the sample is bent or curled and being substantially orthogonalto the y-direction. An electrochemical etching solution 2, such as e.g.a salt/acid aqueous solution, also called electrolyte, is provided in acontainer 8, e.g. a beaker made of a material or coated with a materialthat is not damaged by the electrochemical etching solution, such as forexample glass or plastic. In the example shown, at the bottom of thecontainer 8 a platform 3 is provided. This platform 3 can for examplehave a square shape in a plane parallel to the container bottom and itcan for example be bent in one direction. In the embodiment illustratedin FIG. 1 and FIG. 2, the platform 3 is bent along the x-direction andnot along the y-direction. However, other shapes of the platform 3 canbe used.

As further illustrated, the platform 3 is used to reverse the bendingdirection of a carrying element, e.g. grid 4, carrying the sample 1 tobe etched. This leads to good fixing of the sample between the grid 4and a fixing element, e.g wire 5, and thus to good electrical contact tothe clamped sample 1. Therefore, the shape of the platform 3 may becompatible with the shape of the grid 4.

However, in certain embodiments of the present invention methods otherthan the use of a platform 3 can be used to reverse the bendingdirection of a carrying element 4, e.g. grid. For example, asillustrated in FIG. 5, a flexible carrying element 4, e.g. grid, can beclamped in a frame 10 of smaller dimensions than the carrying element 4such that the carrying element 4 can be in two different stablepositions, each stable position corresponding to a different bendingdirection of the carrying element. The carrying element 4 can be bent ina first direction, e.g. as illustrated for example in FIG. 5( b), whenintroducing the sample 1 in the electrochemical processing solution orwhen removing the sample from the electrochemical processing solution.The carrying element 4 can be bent in a second, different, directionduring electrochemical processing, as illustrated for example in FIG. 5(a). In such an embodiment there is no need for providing a platform 3 inthe container 8. The switching from bending direction, e.g. from thefirst direction towards the second direction, may be triggered by thecarrying element 4 touching the bottom of the container 8. In certainembodiments of the present invention a substantially flat carryingelement 4 can be used, such that there is no need for reversing abending direction.

FIG. 1 shows a system according to an embodiment of the presentinvention during introduction of the sample 1 into the container 8comprising the etching solution 2. FIG. 1( a) shows a cross-sectionalong the y-direction; FIG. 1( b) shows a cross-section along thex-direction. In the example shown, the sample is introduced in such away that the direction in which it is bent is aligned with the directionin which the platform 3 is bent (in the example illustrated thex-direction). The sample 1, in the example shown a cylindrically curledsample 1, is placed on a holder comprising an electrically conductivebendable carrying element, such as a grid 4, e.g. an electricallyconductive grid such as a metallic grid 4. Other electrically conductivematerials may be used for forming the grid 4. In particular embodiments,the curvature of the cylindrical sample has a smaller diameter than thecurvature of the carrying element 4. Instead of a grid 4, any flexibleand electrically conductive substrate may be used. However, a grid 4 isadvantageous as it may improve liquid flows and release of bubbles. Thesample 1 is kept in place on the grid 4 by means of a fixing element 5,for example a linear fixing element e.g. a wire 5. Instead of a wire 5,other fixing elements can be used, such as for example a rod or twosmall fixing elements, one at or close to two opposite extremities ofthe sample. The fixing element can be a rigid element or a flexibleelement or can be made of a material that allows some degree ofdeformation (such as e.g. rubber or foam). The fixing element can beattached to the grid or it can be a separate element. As illustrated inFIG. 1( b), the sample 1 is bent in one direction and the wire 5 isoriented in a direction that is substantially equal to a direction inwhich the sample 1 is not bent. A handler 7 can be attached to theholder, to enable easy handling of the holder comprising the carryingelement 4 and the fixing element 5.

FIG. 2 shows the system during electrochemical etching. FIG. 2( a) showsa cross-section along the y-direction; FIG. 2( b) shows a cross-sectionalong the x-direction. The carrying element 4, e.g. bendable grid 4, ispushed to the platform 3 at the bottom of the container 8, such that itfollows the shape of the platform 3. In particular embodiments thecarrying element, e.g. grid 4, is electrically conductive and providesan electrical contact to the sample 1 to enable electrochemicalprocessing. During electrochemical processing, e.g. etching, theconductive carrying element, e.g. grid 4, and the wire 5 are pushedtowards one another, thereby clamping the sample 1 and ensuring goodelectrical contact to the sample. In the etching solution 2 anelectrode, e.g. negative electrode 6, is provided. The negativeelectrode 6 can for example be in the form of a plate or a grid and ismade of an electrically conductive material, such as for examplestainless steel or any other electrically conductive material that isresistant to the electrochemical process. When the electrode 6 is in theform of a grid, bubbles or gases resulting from the etching process caneasily escape.

FIG. 3 shows the system with the holder and the sample afterelectrochemical etching. A cross section along the x-direction is shown,i.e. along the direction in which the sample was initially bent. In thisparticular example, after removing the metallic layer from the sample,the sample is substantially uncurled. It can be removed from the etchingbath by means of the holder comprising the carrying element 4 and thehandler 7. After electrochemical etching, the etched sample can befragile, for example it can be a thin silicon layer (e.g. a few tens ofmicrometers thick).

In certain embodiments of the present invention the shape of theplatform 3 and the carrying element 4 may for example be rectangular,wherein the platform is bent in one direction. However, other shapes arepossible, such as a circular shape, en elliptical shape, a polygonshape, etc.

Although in FIGS. 1 to 3 a horizontal implementation of a systemaccording to one embodiment of the present invention is shown, otherimplementations are possible. For example, to easily remove compounds,e.g. salts, that may be formed during electrochemical processing, avertical implementation can be used allowing these compounds to fall onthe bottom of the beaker e.g. by gravity. In this case a platform 3would not be provided at the bottom of the container 8 but it could beclamped to the grid holder, as illustrated in FIG. 4.

FIG. 6 shows a flowchart of one embodiment of a method ofelectrochemically processing non-flat samples and/or samples that canchange shape during processing. The method 100 includes, at a block 110,clamping a sample between a grid-shaped carrying element and a fixingelement at at least one point, thus providing an electrical contact tothe sample. Moving to a block 120, the method may further includeelectrochemically processing the clamped sample, while allowing theelectrochemically processed sample to change shape without interruptingthe electrical contact.

The foregoing description details certain embodiments of the invention.It will be appreciated, however, that no matter how detailed theforegoing appears in text, the invention may be practiced in many ways.It should be noted that the use of particular terminology whendescribing certain features or aspects of the invention should not betaken to imply that the terminology is being re-defined herein to berestricted to including any specific characteristics of the features oraspects of the invention with which that terminology is associated.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the technology without departing from the spirit ofthe invention. The scope of the invention is indicated by the appendedclaims rather than by the foregoing description. All changes which comewithin the meaning and range of equivalency of the claims are to beembraced within their scope.

1. A system for electrochemically processing non-flat samples and/orsamples that can change shape during the electrochemical processing, thesystem comprising: a sample holder configured to provide an electricalcontact to the sample during electrochemical processing, wherein thesample holder comprises a grid-shaped carrying element and a fixingelement for clamping the sample in between the fixing element and thecarrying element, thus providing electrical contact to the sample whileallowing the sample to change shape without interrupting the electricalcontact.
 2. The system according to claim 1, wherein the carryingelement is bent in at least a first direction.
 3. The system accordingto claim 2, further comprising a pre-formed platform for, in use,reversing the bending of the carrying element.
 4. The system accordingto claim 1, wherein the carrying element is a flexible carrying elementclamped in a frame of smaller dimensions than the dimensions of thecarrying element such that the carrying element can take on either oftwo stable positions, each stable position corresponding to a differentbending direction of the carrying element.
 5. The system according toclaim 4, further comprising a pre-formed platform for, in use, reversingthe bending of the carrying element.
 6. The system according to claim 1,wherein the fixing element is a linear fixing element.
 7. The systemaccording to claim 1, wherein the fixing element provides a contact areabetween the fixing element and a sample in the form of a point or anarea with limited dimensions less than about 10% of the correspondingdimensions of the sample.
 8. The system according to claim 7, whereinthe contact area between the fixing element and the sample is asubstantially circular contact area.
 9. The system according to claim 1,wherein at least one of the fixing element and the carrying element iselectrically conductive.
 10. The system according to claim 1, furthercomprising a handler for handling the sample holder.
 11. A method ofelectrochemically processing non-flat samples and/or samples that canchange shape during processing, the method comprising: clamping a samplebetween a grid-shaped carrying element and a fixing element at at leastone point, thus providing an electrical contact to the sample; andelectrochemically processing the clamped sample, while allowing theelectrochemically processed sample to change shape without interruptingthe electrical contact.
 12. The method according to claim 11, whereinallowing the electrochemically processed sample to change shapecomprises reversing a bending direction of the carrying element carryingthe sample.
 13. The method according to claim 11, wherein the carryingelement is bent in at least a first direction.
 14. The method accordingto claim 11, wherein the carrying element is a flexible carrying elementclamped in a frame of smaller dimensions than the dimensions of thecarrying element such that the carrying element can take on either oftwo stable positions, each stable position corresponding to a differentbending direction of the carrying element.
 15. The method according toclaim 11, wherein the fixing element is a linear fixing element.
 16. Themethod according to claim 11, wherein the sample is clamped such thatthe fixing element provides a contact area between the fixing elementand the sample in the form of a point or an area with limited dimensionsless than about 10% of the corresponding dimensions of the sample. 17.The method according to claim 16, wherein the contact area between thefixing element and the sample is a substantially circular contact area.18. The method according to claim 11, wherein at least one of the fixingelement and the carrying element is electrically conductive.